catch missing task_id for chgcars

Author: tschaume

mp_api/client/mprester.py

@@ -1347,6 +1347,9 @@ def get_charge_density_from_material_id(
         task_ids = self.get_task_ids_associated_with_material_id(
             material_id, calc_types=[CalcType.GGA_Static, CalcType.GGA_U_Static]
         )
+        if not task_ids:
+            return None
+
         results: list[TaskDoc] = self.materials.tasks.search(
             task_ids=task_ids, fields=["last_updated", "task_id"]
         )  # type: ignore